The method according to the invention is substantially characterised in that the comminuted fiber material is treated in at least one stage prior to said digestion, in the presence of a liquid containing at least one compound having the ability to form complexes together with metals existing naturally in the fiber material. Thus, the treatment with sequestering agent is carried out immediately prior to a pre-impregnation of the chips, for instance, or alternatively during, i.e. simultaneously with the pre-impregnation usually performed before digestion. Treatment with the sequestering agent added is performed so that a pulp is obtained after said digestion process which pulp, besides having a lower content of metals, primarily manganese, has a tearing resistance at least 10% higher, a viscosity at least 5% higher, and produces a yield at least 1% higher than corresponding parameters for a pulp manufactured without said pre-treatment with sequestering agent, calculated within the same kappa number interval.
The invention is applicable to any method whatsoever for manufacturing chemical pulp. A chemical pulp is defined as a pulp having a kappa number below about 100. Such pulps include sulphite and bisulphite pulps based on sodium, potassium or magnesium, alkaline neutral sulphite pulp, pulps of anthraquinone plus hydroxide (NaOH/KOH) or carbonate (Na.sub.2 CO.sub.3 /K.sub.2 CO.sub.3) plus possibly oxygen gas, polysulphide pulp, sulphate pulp and pulp produced by pre-impregnating wood with hydrogen sulphide before alkaline delignification, and also pulps produced by delignification of wood with organic solvent such as methanol, ethanol, possibly in the presence of inorganic solvent.
The compound able to form complexes with metals in the fiber material is suitably selected from the group consisting of non-nitrogenous polycarboxylic acids, nitrogenous polycarboxylic acids and phosphonic acids. Diethylene triamine pentacetic acid (DTPA), ethylene diamine tetracetic acid (EDTA) or nitrilo triacetic acid (NTA) are preferred from the first category, oxalic, citric or tartaric acid from the second category, and diethylene triamine pentaphosphoric acid from the third category. Most preferred are EDTA and DTPA. Two or more of the compounds may also be used, and in any combination whatsoever.
The treatment with sequestering agent is suitably performed at a pH value above about 5.0 and at a liquid/fiber material ratio greater than 2:1, preferably greater than 3:1. According to a suitable embodiment said treatment is performed at a temperature of at least 80.degree. C., preferably at least 100.degree. C., a pressure of at least 2 bar, preferably at least 5 bar, most preferably at least 10 bar, and over a period of at least 20 minutes, preferably at least 40 minutes, most preferably at least 60 minutes.
The sequestering agent is supplied in a quantity suitably within the interval 0.5-10 kg per ton of dry fiber material, preferably 1.5-5 kg and most preferably 2-4 kg per ton of dry fiber material.
A separate treatment vessel may be used for the treatment of the wood with sequestering agent, said vessel being located before, i.e. upstream of the digester tank. The treatment according to the invention may be included with the digestion in a continuous process or a discontinuous process for pulp production. The invention is applicable to all types of continuous and discontinuous digestion methods for the manufacture of chemical pulp.
According to one embodiment of the invention at least a considerable portion of free liquid containing metal complexes formed by said treatment is removed from the wood upon completion of the treatment with sequestering agent. This can be achieved by draining, i.e. thickening, and subsequent washing of the wood with a liquid free from metals or having low metal content. The liquid containing metal complexes is preferably removed by being displaced by cleaner liquid of the type described. The liquid removed is transferred directly to an evaporation system. Alternatively the formed metal complexes are permitted to accompany the fiber material into the digestion process.
At least a part of said liquid present during treatment of the fiber material and containing the sequestering agent, consists of spent liquor, fresh digestion liquid, effluent from bleaching processes, condensation, mains water or lake water, or mixtures thereof. The spent liquor used is suitably the spent liquor having reduced, low content of metals that is obtained at said digestion following said treatment with sequestering agent.
Generally the digestion process includes a pre-impregnation of the wood with digestion liquid and/or spent liquor and according to one embodiment of the invention, the treatment with sequestering agent is performed prior to said pre-impregnation and is followed by a washing stage of suitable type as described above. According to another embodiment the treatment with sequestering agent is performed in combination with the actual pre-impregnation as an integrated treatment, in which case the sequestering agent is preferably added together with the impregnation liquid. In this case the metal complexes formed, together with any excess of sequestering agent remaining, accompany the wood to the digestion zone(s) and are not therefore removed before digestion, but at a later stage when the spent liquor is withdrawn. In certain cases the impregnation phase may be relatively short, such as down to about 1 minute, during which brief period treatment with sequestering agent is performed before the digestion phase is started in the continuous process.
Said spent liquor may be black liquor received from the digestion of wood that has been treated with sequestering agent in accordance with one of the alternatives described above. The cooking liquid may be fresh white liquor.
The treatment with sequestering agent may be most advantageously performed in conjunction with an isothermal cooking process that includes a final extended displacement step in which the operating conditions correspond, or substantially correspond, to those prevailing in the preceding digestion zone(s).
The pulp is delignified with oxygen gas after the digestion process. The pulp is suitably treated with sequestering agent immediately prior to the delignification with oxygen gas. The pulp delignified with oxygen gas may then suitably be bleached with a bleaching agent containing hydrogen peroxide, possibly in combination with ozone and/or peracetic acid.